docs: def E2R(self, array, rlzi):
          """
          :param array: an array of shape (E, ...)
          :param rlzi: an array of E realization indices
          :returns: an aggregated array of shape (R, ...)
          """
          z = numpy.zeros((self.num_rlzs,) + array.shape[1:], array.dtype)
          for a, r in zip(array, rlzi):
              z[self.rlz2idx[r]] += a
          return z